In the past, catalysts such as sulfuric acid or alkoxytitanium have been used for esterification reactions and transesterification reactions.
However, it is difficult to lower the acid value of the end product of the acid catalyst, while the reactivity of the alkoxytitanium is slow.
To improve the reactivity therefore, and easily remove catalyst residues, attempts have been made to further improve this alkoxytitanium catalyst, such as the polyol polytitanate produced by the reaction of an alkoxytitanium and a low molecular weight polyol (Japanese Patent No. 1795216) or the polytitanic acid arising from reaction of alkoxytitanium and water (Japanese Patent No. 1885399).
When this alkoxytitanium is reacted with a water-soluble polyfunctional polyol, a polyol polytitanate is generated, and if this is further reacted with water, polytitanic acid is generated. When the polyol polytitanate or polytitanic acid are used as catalysts, the polytitanate/polytitanic acid reacts with alcohol and water which produces OH groups on the surface and activates them. However, although the OH groups on the surface are activated, they are consumed as the reaction proceeds, the active sites disappear, and the catalyst becomes inactive.
Dioctyl phthalate has been used as a plasticizer for polyvinyl chloride and it has a very high performance, but in outdoor applications, it volatilizes due to its vapour pressure. In this regard, polyesters or complex esters produced from a dibasic acid, diol and monofunctional alcohol can be used as non-volatile plasticizers. They exhibit excellent properties, and are expected to have the same plasticizer capability corresponding to their viscosity.
If is attempted to manufacture this type of ester using a catalyst of the prior art, the acid value does not decrease if there is not much alcohol present in the system. However, if the alcohol is increased in order to accelerate esterification and reduce the acid value, the polymerization degree does not increase. As the reaction is performed using an amount close to the stoichiometric amount to increase the polymerization degree and produce the polyester, the acid value does not decrease and the specification is still one order of magnitude too high even for electrical components which require a low electrical conductivity. Thus, a low acid value polyester was desired. On the other hand, complex esters having a low polymerization degree and a specific molecular weight, have a molecular weight distribution which decreases exponentially as the number n decreases and have a large number of low molecular weight molecules. This leads to a mixture with a large amount of diester with no diol component, and its separation is very difficult. Due to the heat history during separation, it was extremely difficult to improve the shortcoming that the acid value of the component which it is desired to use in the cycle increases.
Thus, a polyester or complex ester which could be used as a plasticizer for polyvinyl chloride could not be manufactured in an actual production process, and the environmental problem that large amounts of plasticizer continue to be released from polyvinyl chloride has still not been resolved.